This invention relates to electron beam moving apparatus for a color cathode ray tube.
Color display systems, such as utilized in color television receivers, include a color cathode ray tube in which three electron beams are modulated by color-representative video signals. The beams impinge on respective color phosphor areas on the inside of the cathode ray tube viewing screen to reproduce a color scene as the beams are deflected to scan a raster. To accurately reproduce the color scene, the three beams must be substantially converged at the screen at all points on the raster. The beams may be converged at points away from the center of the raster by utilizing dynamic convergence methods or self-converging techniques, or a combintion of both. Regardless of the methods utilized to achieve convergence when the beams are deflected, provision must be made to statically converge the undeflected beams in the center region of the screen. Static convergence devices are necessary because the effect of tolerances in the manufacture of electron beam guns and their assembly into the cathode ray tube neck frequently results in a statically misconverged condition.
Some static convergence devices converge the outer beams of three in-line beams of a color cathode ray tube onto the central beam by means of four and six pole rotatable magnetic field ring pairs, producing opposite and like movements, respectively, of the outer beams, such as described in U.S. Pat. No. 3,725,831, by R. L. Barbin entitled "MAGNETIC BEAM ADJUSTING ARRANGEMENTS." Another static convergence device comprises a nonmechanically adjustabe strip or sheath of magnetic material placed about the neck of a color cathode ray tube, such as described in U.S. Pat. No. 4,138,628 by J. L. Smith entitled "MAGNETIZING METHOD FOR USE WITH A CATHODE RAY TUBE." The strip is magnetized to create permanently magnetized regions at appropriate locations and of appropriate polarities and field strengths to produce a static convergence magnetic field. After a color cathode ray tube is statically converged using a magnetized strip, for example, other set-up operations are performed and the catode ray tube is assembled or secured into the television receiver chassis.
After the static convergence operation has been performed, during the remainder of the cathode ray tube set-up or during subsequent television receiver operation, the electron beams may become slightly misconverged. It is desirable to provide a supplemental mechanically adjustable static convergence device to bring the electron beams back into convergence. Because only a small amount of supplemental beam motion is required, typical conventional prior art mechanically adjustable devices may not be sufficiently refined to provide only the small supplemental movements required.
With improved cathode ray tube manufacturing and assembly technique being developed, many tubes may require only a small amount of correcting beam motion, to begin with, to achieve static convergence. The aforementioned supplemental mechanically adjustable static convergence device may then be the only device necessary to achieve convergence. Such a device, if of compact design, may be ideally suited for short-necked cathode ray tubes that have little neck room available on which to mount a static convergence device.